Methods and systems that use suction pick-up devices to manipulate workpieces are frequently used in the processing of sheet metal. In that application, metal sheets have to be fed, for example, from a stack of metal sheets to a processing machine. For this, at least one suction element is applied to the uppermost metal sheet in the stack of metal sheets to form a suction chamber. By generating a vacuum in the suction chamber, the metal sheet and the suction element are fixed to each other. The fixed metal sheet can then be fed, for example by means of a manipulator carrying the suction element, to a sheet metal processing machine. The metal sheets to be handled often have a plurality of through-openings. If a suction element is applied to such a perforated metal sheet, it may be that at least one through-opening opens into the suction chamber so formed. On applying a vacuum to the suction chamber, it is not possible in that case to form in the suction chamber a vacuum sufficient to securely fix the workpiece and the suction element to each other.
DE 36 37 567 describes a method in which a plurality of suction elements are applied to a workpiece having through-openings. By means of a shut-off device of the individual suction elements, only those suction elements which are not situated over a through-opening are connected to a vacuum source. Even with a vacuum generator of conventional capacity, therefore, a sufficient vacuum will be generated at least in the suction chambers situated over a closed surface of the workpiece. The use of this method is generally restricted, however, to workpieces having sufficiently large regions without any through-openings.